Pok?mon Forme Changes are my favorite transformations as far as the poll is concerned. I like having alternate versions of Pok?mon with different appearances and stat builds from their standard form. The only one I don't really like is Meloetta since you have to burn a slot with Relic Song and the form change isn't maintained after a battle so it cuts into its practicality, which is a shame since I like the design of both forms. Things like Deoxys, Rotom, and Shaymin make the game more interesting.
That said, Primal Reversion isn't too bad either. Ever since Omega Ruby & Alpha Sapphire were released and were current, I've harbored an unrealistic wish that the Diamond & Pearl remakes would see Regigigas get a Primal Reversion. Get rid of the moss and change its ability to something other than Slow Start. I'm fond of the Regi trio and Regigigas and am disappointed in how pathetic Regigigas is currently.
Fusion has pretty much just resulted in ugly mash-ups that might be powerful but aren't visually appealing. Which, okay, I get that the fusions we've received so far are just kinda abominations that are forcing things together in an unnatural fashion, but eh. As a fan of Kyurem, I would have preferred that we actually saw the return of the original dragon composed of all three instead of Black Kyurem and White Kyurem.